Techniques for fixturing and measuring optical components that have substantially flat or planar surfaces are well known to those skilled in the optical metrology arts. However, many of the same fixturing techniques used for metrology with flat components can be unsuitable for mounting and measurement of thinner parts that have non-planar surfaces. Thin glass parts, for example, such as those designed for use in hand-held electronic devices and other apparatus, are often molded or otherwise shaped to have non-planar, three-dimensional shapes that are better suited to the contour of the device than are flat shapes. Tolerances for such parts can be demanding, depending upon the particular application. It can be difficult to properly position a curved part in a fixture for optical testing and measurement, without causing some measure of overconstraint that can distort the measured part and thus compromise any measurements made or can even potentially damage the component.
Problems with component fixturing can be compounded with the use of automated testing systems. There can be little tolerance for error in proper positioning of the curved part within the fixture for measurement or for inadvertent movement of the part during translation of the fixture itself. Further, conventional fixtures are formed from metal or other stiff material that is opaque, limiting the light source options for the metrology system.
Repeatability of placement, so that the part being measured seats in only one position within the fixture, is particularly useful for providing smooth, efficient workflow in parts metrology. Achieving repeatability for positioning of thin, non-planar components can be particularly challenging, particularly where there is minimal tolerance for positioning errors.
In parts fixturing for metrology, it is generally necessary to constrain movement of the part from translation along any of the orthogonal x, y, and z axes as well as from rotation about any axis (θx or “pitch”, θy or “roll”, θz or “yaw”). Conventional clamp or vacuum holding techniques may hold the part in position, but are characterized by overconstraint and present the risk of distorting the measured part in some way, leading to inaccurate measurement. Overconstraint problems in fixturing can be further compounded by thermal conditions.
Thus, it can be seen that there is a need for a method and apparatus for improved optical component fixturing, particularly for parts that exhibit some degree of curvature.